In recent years, as one method for copying or printing, the image forming method has been practiced according to the so-called ionography, in which a drum substrate having a dielectric film is used as the dielectric member for receiving electrostatic image, ions are generated by an ion (charged particle) generation means, an electrostatic latent image is formed on the surface of the dielectric member with the ions, the electrostatic image formed is developed with a toner and transfer-fixed onto a transferring material. In the dielectric member for receiving electrostatic latent image to be used for such ionography, as the dielectric layer, a porous anodic aluminum oxide film has been used. Since the film itself of a porous anodic aluminum oxide film has numberless opened micropores, it has such drawbacks as inferior abrasion resistance, low humidity resistance and also image deterioration caused by penetration of toner particles into the pores, etc. Accordingly, there has been proposed the method, in which after formation of the porous anodic aluminum oxide film, adsorption treatment with a silane coupling agent is carried out and then an epoxy resin is impregnated, or an epoxy resin formulated with a silane coupling agent is impregnated (see JP-A-63-294586) (The term "JP-A" as used herein means an "unexamined published Japanese patent application). Concerning the pore sealing treatment of anodic aluminum oxide film, there have also been known the method of impregnating waxes (see JP-A-60-50083), the method of impregnating polytetrafluoroethylene (see JP-A-61-193157), etc.
When an epoxy resin is impregnated after the adsorption treatment with the silane coupling agent as mentioned above or an epoxy resin containing a silane coupling material is impregnated, humidity resistance is almost improved, but the results are still unsatisfactory with respect to surface hardness, hot stress resistance, etc., and also relative dielectric constant is great, about 7 or more, whereby there has been the problem that sufficiently high chargeability cannot be obtained. Further, in the case of these, the resin baking treatment step is required after the impregnation step and also the eliminating treatment step of the resin surface layer thereafter, whereby there have been involved such problems as complication of the steps, lowering in yield on account of such complicated steps, and lowering in reproducibility of characteristics, etc. Also, when waxes or polytetrafluoroethylene were impregnated as the pore sealing material, there have been involved such problems as low chargeability and humidity resistance, or poor adhesion to porous anodic aluminum oxide film which is a dielectric layer, etc.
The present invention has been accomplished in view of the problems as mentioned above in the prior art.